Update README with a worked example of vector constraints

[odow]

Closes #256

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.87%. Comparing base (f32cada) to head (fdba1f5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #257   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files           2        2           
  Lines         778      778           
=======================================
  Hits          777      777           
  Misses          1        1           

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